A. Field of the Invention
The invention relates to improving the accuracy and controllability of a manipulator arm by reducing the variability of the location of its distal end, increasing manipulator arm maximum payload without decreasing accuracy, and assuring straightness of a beam, whether it be cantilevered or spanning two fixed ends.
The fact that beams used in manipulators are not perfectly rigid is a problem that has long plagued robot builders. Traditionally, the location of a manipulator's distal end (usually the gripper or tool), henceforth to be called the endpoint, has been computed from angular measurements taken at each of the joints connecting the manipulator's multiple links. Such a scheme, however, assumes that these links are rigid, i.e. deflections of the links are negligible.
It is known though that these links are not perfectly rigid and that they can bend by significant amounts when subjected to outside forces (including gravity), causing changes in endpoint position without affecting angular measurements taken at the joints. Stated in simpler terms, the manipulator's endpoint is not where the manipulator's controller thinks it is.
In an effort to overcome this problem, some have tried to measure the location of the manipulator endpoint directly rather than through the joint angles. This approach faces several major obstacles:
1. In order to fully determine the position and orientation of an object in space (such as the gripper), six separate measurements must be made; i.e. along three translational axes and three rotational axes.
2. Some provisions must be made for tracking of the endpoint by the measurement device in spite of the many possible obstacles between the base and the endpoint. If the measurement path is made to follow the links of the manipulator, the number of measurements will be six times the number of links. Given the complexity involved in such measurements, such a system, if it were to be developed, would be extremely expensive.
3. Assuming, nevertheless, that such a measurement system were employed, there remains the task of making the manipulator able to quickly and predictably make use of this information by adjusting the neccessary joint angles in such a way as to place the endpoint in the required position and orientation. Since all the links would be subjected to bending deformation, and since cantilever bending causes a rotation as well as a translation of the disital end, the required corrective action would be very computation intensive and would neccessitate rapid motion from all actuators. It is not likely that such action could be taken without creating a significant time delay.
B. Prior Art
Flemming, in his U.S. Patent, disclosed a system designed to attack the problem in a manner similar to that described above. However he made no provision for the actuation scheme required to carry out the intended motion.
Klein and Nachtigal, in their paper "The Application of Active Control to Improve Boring Bar Performance," in the June, 1975 edition of Journal of Dynamic Systems, Measurement and Control beginning at p. 179, described their successful experiment in which a thin cantilever bar was controlled in one degree of freedom at its base based on position information obtained at the endpoint. However their idea would suffer from all the problems described above if it were to be expanded to a multidegree-of-freedom manipulator.